Pregnancy and Working Out

I am 6 months pregnant with my second child.  Today I go in for my first ultrasound and know that coming out of it I will finally know this is real.  I wasn't prepared to be having a child so soon after my first.  Sure it's been three years but in my mind I pictured having my life under control, my finances steady, my son heading off to kindergarten before I even broached the subject of another child with my husband. 
I was smack dab at the beginning of getting my life under control and getting back in shape...for me...I was doing well with  a few steps back here and there.  At the beginning of this pregnancy I weighed in at 148 or was that 158??  I'll have to check, 158 sounds right and it was where I was plateauing.  I bounced between 158 and 162 it seemed almost daily.  I wish I knew what I needed to do to get back on track and get that number going down.  I thought I was getting there until come March 7th and my monthly visitor failed to appear. 
I didn't let that get me down though surprisingly.  The first few months I maintained that 158 mark, watching what I ate more closely to make sure I was getting the nutrients I needed for my child as pre-natal vitamins make me so so sick to my stomach.  I upped my fruit intake, I crave fruit anyways and I upped my fresh vegetable intake. Makes me wonder if I wasn't pregnant what this little change could have done for my weight loss.
My problem now is exercise.  I try to stretch real good every morning and  take the dogs out for long walks before the heat explodes usually by 9am.  The afternoons are a little tougher as the heat is just obnoxious and sends me skittering inside every time I open the front door or even think about getting in my car to go anywhere.  I want to try yoga, but it is difficult in a small town to find someone that knows how to shape a program for a pregnant woman.  Everyone says just take it easy on exercise and drink plenty of fluids, which I do but I feel I could be and should be doing more to prepare my body.  I didn't do much with my son and though labor with him was not difficult, I feel it could have gone better for me  if I had prepared more physically.